Communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM) network. The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for a user equipment. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for the third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in the UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a Radio Network Controller (RNC) or a Base Station Controller (BSC), which controller node supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3GPP and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface.
In the 3GPP LTE network, base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
The 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Multi-antenna techniques may significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems.
In the 4G communications network, the EPS, an Automatic Neighbour Relation (ANR) function and an X2-C Transport Network Layer (TNL) address discovery were introduced to allow the eNBs that have not yet setup an X2 interface between each other to do so. This may for example be the case when the eNB has determined that a neighbour eNB serves a cell suitable as a target cell for handover. Exchange of addresses between the eNBs, e.g. exchange of the respective eNB's X2-C interface TNL signalling address, may be achieved by means of signalling via the core network, see 3GPP TS 36.300. Signalling between the eNBs via the core network is enabled due to the fact that each eNB has established a connection towards a Core Network (CN) node, e.g. an S1-MME interface instance towards a Mobility Management Entity (MME), for signalling purposes. Thereby, each eNB provides its node identity to the MME.
The TNL establishes physical and logical connections between the RAN and the CN, e.g. between a Radio Network Node (RNN) such as an eNB and a CN Node (CNN). The TNL comprises the transport network Control plane (C-plane) and the transport network User plane (U-plane).
An eNB node identity (ID) is derived from the 20 Most Significant Bits (MSBs) of the identities of the E-UTRA cells it serves. An E-UTRA cell ID is 28 bits long. However, it should be understood that the eNB identities may be constituted from less or more than the 20 MSBs of the identities of the E-UTRA cells, e.g. for Home eNBs or for eNBs with flexible eNB ID, as introduced in the 3GPP Release 14 (Rel-14).
An eNB detecting a cell of a neighbouring eNB suitable e.g. as a handover target, will request the UE reporting the cell to provide the E-UTRA cell identity. The neighbouring eNB identity may be derived from the E-UTRA cell identity.
If an eNB, e.g. a requesting eNB, wants to setup an X2 interface to a neighbour eNB serving a potential handover target cell, the requesting eNB provides its node identity to the CN node, e.g. the MME, by means of S1-MME signalling, e.g. by means of an eNB CONFIGURATION TRANSFER. Thereby, asking the MME to resolve the eNB identity of the node hosting the potential handover target and relay the request for providing the X2-C signalling address to the neighbour eNB. Once the requesting eNB has received the X2-C signalling address, the X2-C interface may be setup.